Operation of lean burn engines, e.g., diesel engines and lean burn gasoline engines, provide the user with excellent fuel economy, and have very low emissions of gas phase hydrocarbons and carbon monoxide due to their operation at high air/fuel ratios under fuel lean conditions. Diesel engines, in particular, also offer significant advantages over gasoline engines in terms of their durability, and their ability to generate high torque at low speed. Effective abatement of NOx from lean burn engines is difficult to achieve because high NOx conversion rates typically require reductant-rich conditions. Conversion of the NOx component of exhaust streams to innocuous components generally requires specialized NOx abatement strategies for operation under fuel lean conditions.
One such strategy for the abatement of NOx in the exhaust stream from lean burn engines uses NOx storage reduction (NSR) catalysts, which are also known in the art as “lean NOx trap (LNT)”. NSR catalysts contain NOx sorbent materials capable of adsorbing or “trapping” oxides of nitrogen under lean conditions and platinum group metal components to provide the catalyst with oxidation and reduction functions. In operation, the NSR catalyst promotes a series of elementary steps which are depicted below in Equations 1-5. In an oxidizing environment, NO is oxidized to NO2 (Equation 1), which is an important step for NOx storage. At low temperatures, this reaction is typically catalyzed by the platinum group metal component, e.g., a platinum component. The oxidation process does not stop here. Further oxidation of NO2 to nitrate, with incorporation of an atomic oxygen, is also a catalyzed reaction (Equation 2). There is little nitrate formation in absence of the platinum group metal component even when NO2 is used as the NOx source. The platinum group metal component has the dual functions of oxidation and reduction. For its reduction role, the platinum group metal component first catalyzes the release of NOx upon introduction of a reductant, e.g., CO (carbon monoxide) or HC (hydrocarbon) (Equation 3) to the exhaust. This step recovers NOx storage sites but induces release of NOx species. The released NOx is then further reduced to gaseous N2 in a rich environment (Equations 4 and 5). NOx release can also be induced by reductants in a net oxidizing environment. However, the efficient reduction of released NOx by CO requires rich conditions. A temperature surge can also trigger NOx release because metal nitrate is less stable at higher temperatures. NOx trap catalysis is a cyclic operation. Metal compounds are believed to undergo a carbonate/nitrate conversion, as a dominant path, during lean/rich operations.
Oxidation of NO to NO2 NO+½O2→NO2  (1)
NOx Storage as Nitrate2NO2+MCO3+½O2→M(NO3)2+CO2  (2)
NOx ReleaseM(NO2)2+2CO→MCO3+NO2+NO+CO2  (3)
NOx Reduction to N2 NO2+CO→NO+CO2  (4)2NO+2CO→N2+2CO2  (5)
In Equations 2 and 3, M represents a divalent metal cation. M can also be a monovalent or trivalent metal compound in which case the equations need to be rebalanced.
While the reduction of NO and NO2 to N2 occurs in the presence of the NSR catalyst during the rich period, it has been observed that ammonia (NH3) can also form as a by-product of a rich pulse regeneration of the NSR catalyst. For example, the reduction of NO with CO and H2O is shown below in equation (6).
Reduction of NO to NH3 CO+H2O→H2+CO2  (6a)2NO+3H2+2CO→2NH3+2CO2  (6b)
This property of the NSR catalyst mandates that NH3, which is itself a noxious component, must also now be converted to an innocuous species before the exhaust is vented to the atmosphere.
An alternative strategy for the abatement of NOx under development of mobile applications (including treating exhaust from lean burn engines) uses selective catalytic reduction (SCR) catalyst technology. The strategy has been proven effective as applied to stationary sources, e.g., treatment of flue gases. In this strategy, NOx is reduced with a reductant, e.g., NH3, to nitrogen (N2) over an SCR catalyst that is typically composed of base metals. This technology is capable of NOx reduction greater than 90%, thus it represents one of the best approaches for achieving aggressive NOx reduction goals.
Ammonia is one of the most effective reductants for NOx at lean condition using SCR technologies. One of the approaches being investigated for abating NOx in diesel engines (mostly heavy duty diesel vehicles) utilizes urea as a reductant. Urea, which upon hydrolysis produces ammonia, is injected into the exhaust in front of an SCR catalyst in the temperature range 200-600° C. One of the major disadvantages for this technology is the need for an extra large reservoir to house the urea on board the vehicle. Another significant concern is the commitment of operators of these vehicles to replenish the reservoirs with urea as needed, and the requirement of an infrastructure for supplying urea to the operators. Therefore, less burdensome and alternative sources for supplying the reductant NH3 for the SCR treatment of exhaust gases are desirable.
Emissions treatment systems that utilize the catalytic reduction of NOx in the exhaust to generate NH3, in place of an external reservoir of NH3 or NH3 precursor have been described, but these systems have limitations. In such systems, a portion of the NOx component of the exhaust is used as an NH3 precursor in such systems. For instance, U.S. Pat. No. 6,176,079 discloses a method for treating an exhaust gas from a combustion system that is operated alternately in lean and rich conditions. In the method, nitrogen oxides are intermediately stored during lean operation, and released during rich operation to form NH3 that is stored. The stored NH3 can be released, and thereby reduce nitrogen oxides during a subsequent lean operation.
European Patent Publication No. 773 354 describes a device for purifying the exhaust gas of an engine that has a three way catalyst connected to an NH3 adsorbing and oxidizing (NH3-AO) catalyst. The engine is operated with alternating rich and lean periods. During a rich operation the three way catalyst synthesizes NH3 from NOx in the inflowing exhaust gas, and the NH3 is then adsorbed on the NH3-AO catalyst. During the lean operation NOx passes through the three way catalyst and the adsorbed NH3 is desorbed and reduces the inflowing NOx.
International Published Patent Application WO 97/17532 discloses a method and device for purifying the exhaust gas from an engine, and in particular, describes a method and device for purifying NOx in the exhaust gas. In one embodiment, the publication describes a device that has a three way catalyst upstream of, and on the same carrier as a NOx occluding and reducing catalyst. Downstream of the NOx occluding and reducing (NOx-OR) catalyst is a NH3 adsorbing and oxidation (NH3-AO) catalyst. To prevent any NH3 breakthrough, the device is also equipped with a NH3 purifying catalyst downstream of the NH3-AO catalyst. The air/fuel ratio of the cylinders of the engine are alternately and repeatedly rendered lean and rich to thereby render the exhaust gas air/fuel ratio, alternately and repeatedly rich and lean.
In the method described for this device in the WO97/17532 publication, when the air/fuel ratio of the exhaust gas is lean, NOx passes through the three way catalyst, and NOx is occluded in the NOx-OR catalyst. It is described that any NOx passing through the NOx-OR catalyst is purified in the following NH3-AO catalyst. NH3 is desorbed from the NH3-AO catalyst when the air/fuel ratio of the exhaust gas is lean, and the desorbed NH3 reduces the NOx.
When the air/fuel ratio of the exhaust gas is rich, a part of the NOx in the exhaust gas is converted to NH3 in the three way catalyst. The NH3 then passes into the NOx-OR catalyst, where the NOx is released, reduced and purified by the inflowing NH3. Any NH3 passing through the NOx-OR catalyst that is not consumed by the reduction of NOx is adsorbed on the NH3-AO catalyst, or is purified further downstream in the NH3 purifying catalyst.
A problem associated with methods that utilize a portion of the NOx in the exhaust gas as an NH3 precursor is that, depending on operating conditions, NH3 generated during rich operating periods often is insufficient to treat the NOx during lean periods (i.e., when the exhaust gas composition has a λ>>1). This deficiency can limit the range of operating conditions where NOx can be effectively treated by other emissions treatment systems.
As the conditions that emission treatment systems operate under vary for different vehicles powered by lean burn engines, flexible approaches for the design of emission treatment systems are needed to achieve ever more stringent requirements for NOx abatement. In particular, approaches that account for the effect on NOx storage and NH3 formation during lean and rich periods of operation of altering the NSR catalyst composition offer more reliable and practical pathways to achieving this goal.